Method and apparatus for developing a latent magnetic image

ABSTRACT

A method and apparatus for developing a latent magnetic image in which a layer of magnetically attractable toner powder having a specific resistance less than 10 9  ohms.meter is conveyed by a toner conveyor to a developing zone past a medium carrying the latent image and an AC voltage is applied between the toner conveyor and the medium carrying the latent image.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging system and more specificallyto a method and apparatus of developing a latent magnetic image.

2. Discussion of Related Art

U.S. Pat. No. 4,368,687 describes a method and apparatus for developinga latent magnetic image, in which a uniform, very thin layer of anelectrostatically charged insulating and magnetically attractable tonerpowder is applied to a toner conveyor and the thin layer of toner powderis fed to a developing zone where it is brought to a distance of 200 to400 micrometers from the image-carrying medium. To complete the imagedevelopment, an AC voltage is applied in the developing zone between thetoner conveyor and the image-carrying medium.

In comparison with other known developing methods in which magneticallyattractable toner powder is brought into (frictional) contact with animage-carrying medium in a developing zone, the method according to theinstant U.S. patent has the advantage that background resulting from thedeposition of toner particles on the image-free parts of the medium isprevented. A disadvantage of this method, however, is that the apparatusfor performing the method must satisfy high accuracy requirements toachieve the required slit width such that the apparatus parameters mustbe strictly adhered to. In addition, thin toner layers have to be usedin this method so that during the development of images with highinformation density, such as large black areas, the disadvantage mayarise such that the toner supply in the developing zone may be too lowand the developed images consequently have an inadequate density.

SUMMARY OF THE INVENTION

Therefore, it is an object to the present invention to provide a methodand apparatus for developing a latent magnetic image which will overcomethe above-noted disadvantages.

It is another object of this invention to provide a method wherebygood-quality images are obtained performed with an apparatus which doesnot have to meet rigorous tolerance requirements.

A further object of the present invention is to provide a latentmagnetic imaging process which has a broad range of tolerances.

The foregoing objects and others are accomplished in accordance with thepresent invention, generally speaking, by providing a layer of amagnetically attractable toner powder on a toner conveyor which feedsthe toner powder to a developing zone past a medium carrying a latentmagnetic image. An AC voltage is applied between the toner conveyor andthe latent image-carrying medium so as to selectively develop the latentmagnetic image. Characteristic of the present invention is that themagnetically attractable toner powder used has a specific electricalresistance of less than 10⁹ ohms.meter.

The present invention also provides a magnetic printing apparatus forperforming the method herein defined comprising a magnetizable imagerecording medium, means for recording a latent image on the imagerecording medium, a toner conveyor to convey magnetically attractabletoner powder past the image recording medium in a developing zone, ametering device for metering a layer of magnetically attractable tonerpowder on the toner conveyor, and means for generating an AC voltage inthe developing zone between the toner conveyor and the image recordingmedium. The magnetographic apparatus is characterized in that in thedeveloping zone the shortest distance "A" in mm between the tonerconveyor and the surface of the image-recording medium is determined tobe between

    B+0.6<A<B+1.6

where "B" is the distance in mm between the metering device and thesurface of the toner conveyor.

In the method according to the instant invention, very good qualityimages are obtained which are free of background. Magnetic imagesdepicted on the image-recording medium with a high resolution of, forexample, 400 dpi (dots per inch) are developed free of background, whileone-pixel lines and loose image pixels can still being distinguishlyreproduced. This good image quality is achieved not only at a relativelylow speed of advancement of the image-recording medium, but also at ahigh speed of advancement of 30 meters per minute, equivalent to aprinting speed of more than 100 pages of A4 paper per minute. Animportant advantage of the method according to the present invention isthat there is a wide working range even when developing high resolutionimages and at high speeds of advancement of the image-recording medium,so that the developing apparatus is not required to satisfy highaccuracy requirements. In the method according to the present invention,it is possible to form on the toner conveyor a layer of toner powderwhich is considerably thicker than the layer thickness indicated in theabove-mentioned U.S. Pat. No. 4,368,687.

An additional advantage of the method according to the present inventionis that the toner powder does not have to be electrostatically chargeprior to the image development, thus obviating quality variationsresulting from irregularities in the charging of the toner powder. As isalready known, the electrostatic and, in particular, the tribo-electriccharging of a toner powder is influenced by varying ambient conditionsof temperature and humidity.

The method according to the present invention uses a magneticallyattractable toner powder having a specific electric resistance of lessthan 10⁹ ohms.meter. The toner powder comprises resin particles in whichmagnetically attractable material is finely distributed. Themagnetically attractable material may be soft or remanent magnetic andcan be selected from those materials known per se for use in tonerpowders. Typical such magnetically attractable material includes iron,carbonyl iron, nickel, chromium dioxide, gammaferrioxide and ferrites ofthe formula MFe₂ O₄ in which M represents a bivalent metal e.g. iron,manganese, nickel, or cobalt or a mixture of metals of other valency.Other examples are the rare-earth iron garnets of the formula R₃ Fe₅ O₁₂in which R denotes a rare-earth or other trivalent ion e.g. Y or Sc. Theiron in these garnets can be partially replaced by another ion or ions.The magnetically attractable material content is of the order ofmagnitude conventional for toner powders and is, for example, 6-20% byvolume for soft magnetic material and 1-10% by volume for remanentmagnetic material.

In addition to magnetically attractable material the resin particlescontain electrically conductive material to give the toner powder aspecific electric resistance of less than 10⁹ ohms.meter. Theelectrically conductive material which, for example, may consist of finecarbon particles or metal particles, such as silver or copper particles,may be finely distributed in the resin particles or deposited on thesurface of the resin particles in a quantity sufficient to render thetoner powder the required specific resistance of below 10⁹ ohms.meter.Preferably, the electrically conductive material is deposited on thesurface of resin particles. Suitable toner powders for use in the methodaccording to the present invention are described, inter alia, inNetherlands Patent Application 7203523. Representative examples of suchtoner powder include powders of which the individual particles consistof a magnetically attractable core composed of about 50 to 95% by weightof thermoplastic resin binder and about 5 to 50% by weight ofmagnetizable material which is finely distributed in the resin binder,and, adhered to the surface of the core and/or partially embeddedtherein, a finely divided conductive material, such as fine carbonparticles, in an amount sufficient to impart the desired conductivity tothe particles.

The resin binder may be a resin well-known in the art of toner powdermanufacture, such as epoxy resin, polyester resin, in particular thepolyester resins derived from bisphenol A or an oxyalkylated derivativethereof and a dicarboxylic acid such as maleic or fumaric acid,polystyrene, polyacrylics and polyvinylcyhloride.

The specific resistance of the toner powder is measured as follows. Acylindrical container having an inside diameter of 17.2 mm, a base whichconsists of brass having a thickness of 1.5 mm, and a wall whichconsists of Teflon having an internal height of 22.9 mm, and a thicknessof 9 mm, is filled with an excess of powder. The filling is thencompressed by crushing it ten times in a crusher made by EngelsmannA.G., of Ludwigshaven, Germany. This filling procedure is repeatedtwice. Excess powder is then wiped off with a ruler and a brass lidhaving a diameter of 17.2 mm and a mass of 55 g is placed on the columnof powder. The filled container is placed in a Faraday cage and a 10volt D.C. is applied between the base and lid. The current intensity ismeasured after about 20 seconds. The measuring procedure (containerfilling and current measurement) is repeated three times, whereafter theaverage current intensity of the three measurements is calculated.

The resistance of the powder follows from the formula: ##EQU1## where:U=the applied voltage (10 volts)

A'=contact area of lid and powder column (2.32×10⁻⁴ m²)

h=height of powder column (2.29×10⁻² m)

Ig=average current strength (in amps).

The specific resistance of the toner powder should be less than 10⁹ohms.meter. No critical bottom limit has been found for the resistance.Thus good image development was obtained even with toner powder having aspecific resistance of 10³ to 10⁴ ohms.meter, which also was found toprovide a wide working range.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its advantages are now explained in greater detailwith reference to FIG. 1 which diagrammatically illustrates a magneticprinting apparatus in which the method according to the instantinvention is used.

DETAILED DISCUSSION OF THE INVENTION

The apparatus according to the present invention comprises a cylindricalimage recording medium 1 consisting of a drum of copper or copper-platedaluminum, the surface of which is covered with a galvanically appliedcobalt-nickel phosphorus layer about 8 micrometers thick, which has amagnetic coercivity of about 77 kA/m. The image-recording medium 1 canrotate in the direction indicated by the arrow. Disposed consecutivelyalong the rotational path of the image recording medium 1, as consideredin the direction of rotation there is a magnetic head array 2, withwhich a latent magnetic image having a resolution of about 400 dpi canbe recorded in the magnetizable layer, a developing device 3, an imagetransfer device 4, a cleaning device 5, and an erase device 6. Themagnetic head array 2 is of the type described in detail in EuropeanPatent Application 87200230. The developing device 3 comprises areservoir 7 for the toner powder 20, a powder supply roller 8 having arough surface, a toner conveyor 9 which feeds the toner powder 20 intothe developing zone 13, and a metering device 10. The powder supplyroller 8 feeds the toner powder to the toner conveyor 9. The latterconsists of a magnetic roller having a rotatable electrically conductivenon-magnetizable sleeve 11 of, for example, copper and a stationarymagnet system 12 inside the sleeve 11. In the embodiment illustrated,the magnet system 12 comprises eight magnet poles magnetized as shown inthe drawing. The magnet pole situated opposite the image-recordingmedium 1 generates a magnetic induction of, for example, about 225 gaussat the surface of sleeve 11 immediately thereabove, while the othermagnet poles generate an induction of 800 gauss at the surface of thesleeve. The lower magnetic induction in the developing zone 13 is of noessential importance to obtaining good image development. All that isimportant is that there should not be such a magnetic field in thedeveloping zone 13 which creates an erasing effect on the latentmagnetic image on the image-recording medium 1. The method according tothe present invention can also be performed using a toner conveyor soconstructed that no magnetic field, or only a very weak magnetic field,is present in the developing zone 13. Embodiments of such tonerconveyors are indicated in U.S. Pat. No. 4, 368,687 mentioned above.

The distance between the sleeve 11 and the image-recording medium 1 canbe varied by moving the toner conveyor 9. The distance between sleeve 11and the metering device 10, which consists of aluminum for example andis in the form of a ruler, is adjustable. The distance "B" as discussedabove, denotes the shortest distance between the sleeve 11 and themetering device 10.

The electrically conductive sleeves of the toner conveyor 9 and theimage-recording medium 1, respectively, are connected to an AC supply14.

A powder image developed on the image-recording medium 1 is transferredto an image-receiving material 21 by the transfer device 4. The latteris a two-step transfer device known per se, in which the powder image ofthe image-recording medium 1 is first transferred, by pressure, to abelt 15 bearing a silicone rubber surface covering. The belt 15 isheated by heating means (not shown) to soften the powder imagetransferred thereto. In a pressure zone formed between the belt 15 and abiasing roller 16 the softened powder image is then transferred to andfixed o the receiving material 21 fed to the pressure zone from a supply(not shown).

EXAMPLES

The working range of the developing device of the instant illustrationis determined by using a toner powder having a resistance of about3.5×10⁵ ohms.meters, a particle size of between 10 and 20 micrometers,and particles containing 20% by volume of a soft magnetic pigment (typeBayferrox B 318 M made by Bayer AG, Germany) and 80% by volume of apolyester resin, the surface being covered with carbon particles.Background-free images of good quality are obtained with the followingsettings:

Speed of rotation of sleeve 11: 25 to 45 meters per minute;

Distance "B" between sleeve 11 and metering device 10: 0.6-2 mm;

AC voltage of 500-2000 V; and frequency 900-2400 Hz.

Primarily dependent on the value of the applied AC voltage, the optimalvalue for the distance A (distance between sleeve 11 and image-recordingmedium 1) was found to be between the distance B plus 0.6 to 1.6 mm. Inthe low area of the AC voltage range (500 to about 900 V), thedifference between distance A and distance B (hereinafter indicated bydelta w) was in the range from 0.6-1 mm, and gradually shifted to highervalues with increasing AC voltage. The delta w appeared to have aworking range of some tenths of a millimeter for each applied Acvoltage. With AC voltages of from about 1200 V to about 600 V thisworking range was determined to be 0.3 to 0.4 mm. With a distance B ofabout 1.3 mm, an AC voltage of about 1500 V and a frequency of 1800 Hz,the value of delta w, given a speed of rotation of sleeve 11 between 25and 45 meters per minute and a speed of advance of the image-recordingmedium 1 of from 15 to 40 meters per minute, was found to be betweenabout 0.9 to 1.3 mm.

In a subsequent test series, the resistance of the toner powder usedvaried between about 10³ and 10⁹ ohms.meter with the above settingsbeing distance B: 1.3 mm; AC voltage 1500 V, 1800 Hz; speed of rotationof sleeve 11: 45 meters per minute; and speed of rotation ofimage-recording medium 1: 15 meters per minute. Good quality prints areobtained in every case with delta w values between 0.9 and 1.3 mm. Thequality of the images obtained with the toner powder having a specificresistance of more than 10⁸ ohms.meter is a fraction less satisfactorythan that of the images obtained with the other toner powders. The tonerpowders used in these tests consisted of particles containing 20% byvolume of soft magnetic pigment (Bayferrox B 318 M) and 80% by volume ofthe polyester resin, the surface being covered with fine carbonparticles.

Using a toner powder in which the particles ranged in size of between 10and 20 micrometers and consisted of 94% by volume polyester resin, 3% byvolume remanent magnetic pigment (type Bayferrox 8140 made by Bayer AG,Germany) and 3% by volume carbon, and which were covered with carbon toa specific resistance of 2×10⁵ ohms.meter, a same working range wasfound as described above for toner powder containing 20% by volume ofthe soft magnetic pigment.

In the method and apparatus according to the present invention, thedistance between the toner conveyor and the surface of theimage-recording medium in the developing zone can be so widely variedthat toner powder 20 is deposited on the image-recording medium onlywhen the AC voltage is applied across the developing zone. If furtherimage development is to be avoided for some reason, e.g. in the event ofa malfunction in the image transfer device or in the supply of imagereceiving material, immediate response is possible by switching off theAC supply. Using the method according to the present invention, amulti-color printing apparatus can be configured in a relatively simplemanner of the type in which a number of developing devices, e.g. 2, 3 or4, are disposed around the rotational path of the image-recordingmedium, each such developing device being filled with toner powder of aspecific color and the appropriate color separation images printed inconsecutive rotational cycles of the image-recording medium, theseparation images being combined in register on a combining medium, e.g.the image-receiving material or an intermediate. The development of eachof the separation images in the associated color is controlled byapplying the AC voltage to the developing device required to beoperative.

Mechanical means to move the developing devices between an operativeposition and an inoperative position or to cut off the toner supply tothe developing devices which are not allowed to be operative are thusunnecessary, so that it is possible to embody an apparatus of simpleconstruction.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to on skilled in the art are intendedto be included within the scope of the following claims.

I claim:
 1. A method of developing a latent magnetic image whichcomprises:forming a latent magnetic image in the surface of amagnetizable image recording medium, providing a layer of a magneticallyattractable toner powder on a surface of a toner conveyor, introducingsaid layer of magnetically attractable toner powder into a developingzone opposite said latent magnetic image, and applying an AC voltagebetween said toner conveyor and said image support medium so as todevelop said latent magnetic image with said magnetically attractabletoner powder to produce a powder image, said toner powder having anelectrical resistance of less than 10⁹ ohms.meter.
 2. The methodaccording to claim 1, further including the step of providing a meteringmeans for metering said layer of magnetically attractable toner powderto a desired thickness on the surface of said toner conveyor.
 3. Themethod according to claim 1, wherein in said developing zone a shortestdistance A in mm between said toner conveyor and said surface of saidimage recording medium comprises

    B+0.6<A<B+1.6

where B is a distance in mm between said metering device and saidsurface of said toner conveyor.
 4. The method of claim 3, wherein saiddistance A is varied by moving said toner conveyor with respect to saidimage recording medium.
 5. The method according to claim 1, wherein saidtoner powder comprises resin particles in which magnetically attractablematerial is finely distributed.
 6. The method according to claim 4,wherein said toner powder further includes electrically conductivematerial in an amount sufficient to impart said required electricalresistance.
 7. The method according to claim 1, wherein distance betweensaid toner conveyor and the surface of said image recording medium issuch that toner transfer to said image recording medium to develop saidlatent magnetic image takes place only when said AC voltage is applied.8. A magnetic printing apparatus for producing a powder imagecomprising:a magnetizable image recording medium; means for recording alatent magnetic image on a surface of said image recording medium; atoner conveyor means for conveying a magnetically attractable tonerpowder past said image recording medium thereby creating a developingzone between said image recording medium and said toner conveyor meansfor said latent magnetic image; a metering device for metering a layerof magnetically attractable toner powder to a desired thickness on asurface of said toner conveyor; and means for generating an AC voltagein said developing zone between said toner conveyor and said imagerecording medium, such that a shortest distance A in mm between thesurface of said toner conveyor and the surface of said image recordingmedium in said developing zone is between B+0.6 and B+1.6, where B is adistance in mm between said metering device and the surface of saidtoner conveyor.
 9. The apparatus according to claim 8, wherein saidtoner conveyor means consists of a magnetic roller including a rotatableelectrically conductive, non-magnetizable sleeve and a stationary magnetsystem disposed inside said sleeve.
 10. The apparatus according to claim8, wherein said means for recording a latent magnetic image on saidimage recording medium consists of a magnetic head array.
 11. Theapparatus according to claim 8, further including a transfer means fortransferring said powder image from said image recording medium to animage receiving material.